A 1951 photograph of the Ratzinger family shows Georg Ratzinger, back row, left, and Joseph after their ordination. Seated, from left, are Maria Ratzinger, mother Maria and father Josef. Ersbistum Muenchen und Freising via AFP/Getty Images

Church law does allow for a pope to resign — if he is of sound mind and not forced out by fear or fraud. Under canon law, if a pope chooses to resign, no one is allowed to tell him he can't.

According to the Vatican, John Paul II had secret letters, in 1989 and in 1994, offering to resign if he were too incapacitated to fulfill his ministry.

Pope Benedict said in Light of the World, a collection of interviews with a German journalist, that a pope should resign if he "clearly realizes that he is no longer physically, psychologically, and spiritually capable of handling the duties of his office, then he has a right and, under some circumstances, also an obligation to resign."

But if it is simply a staggering burden to be pope, Benedict said, "One must stand fast and endure the situation. That is my view. One can resign at a peaceful moment or when one simply cannot go on. But one must not run away from danger and say someone else should do it."

Pope John Paul II quashed speculation that he might choose to resign as his health deteriorated late in life, saying that if it was God's will that he leave the papacy then God would "call him home."

The last pope to resign was Gregory XII in the 1415. But the last uncontested pope to resign was Celestine V, more than 120 years earlier, in 1294.

Gregory's resignation was a relatively uncontroversial political move made at the request of church officials, during a time when there were three claimants to the Chair of St. Peter. His decision to step down was the first step in a series of compromises that eventually ended the Great Schism, and consolidated power under a single pope, Martin V.

The case of Celestine, on the other hand, was extremely controversial. His successor, Boniface VIII, imprisoned him in unsanitary conditions in a castle near Naples, where he died of disease less than a year later. He became so despised that when Italian poet Dante Alighieri wrote The Divine Comedy 25 years later, he placed Celestine in Hell's antechamber: "I saw and recognized the shade of him / Who by his cowardice made the great refusal."

The College of Cardinals, which has been appointed entirely by John Paul and Benedict, will select the next pope in a secret conclave, where Benedict will obviously hold great sway. It is not clear what Benedict will do in the first post-papal retirement in nearly six centuries.

Popes who have resigned — often during times of political turmoil:

Pontian (230-235). Allegedly resigned after being exiled to the mines of Sardinia.

Marcellinus (296-304). Abdicated or was deposed after complying with Roman Emperor Diocletian's order to offer sacrifice to pagan gods.

Martin I (649-655). Exiled by Emperor Constans II to Crimea.

Benedict V (964). Elected after the assassination of the prior pope, he was pushed out a month later, by the emperor, who favored a different candidate.

Benedict IX (1032-45). Resigned after selling the papacy to his godfather Gregory VI.